Description of the Prior Art
The desire to separate, at acceptable cost, the components of brackish water and of sea water has existed for many years. Considerable technology now exists which is capable of purifying brackish water, sea water and other saline water.
One known technique of desalination is the electrodialysis process in which the ions forming the salt are pulled by electric forces from the saline water through membranes and thereafter concentrated in separate compartments. Another known technique of desalination is the reverse osmosis process which relies on the use of high pressure to force relatively salt-free water through membranes thereby separating relatively salt-free water from the initial saline water. In addition, brine concentrators, such as Resources Conservation Company brine concentrators (R.C.C. brine concentrators) are used to separate water and salt. In addition, solar ponds (both non-covered and covered) and molten salt systems are known methods of separating components of aqueous mixtures.
Each of these techniques relies on energy to provide the means for separating the water from the salt. For example, in the electrodialysis process, the ions forming the salt are "pulled out" of the saline water by electric forces and concentrated in separate compartments. In general, the higher the salinity of the saline water, the greater the amount of electric energy needed for this process per unit of fresh water product. Generally, at very high salt concentrations conventional electrodialysis should not be employed. For example, with some reverse osmosis membranes it is possible to obtain from saline water a reasonable yield of product water containing less than 500 ppm of dissolved salts. The salts contained in the saline water are substantially rejected by the membrane and are concentrated in a reject brine. The reverse osmosis process necessarily requires energy. The amount of energy required and the yield of product water produced from saline water are directly related to the salt concentration of the saline water being processed and, within limits, to the temperature of the saline water. In general, reverse osmosis units operate less efficiently and require more energy per unit of water product as the salt concentration of the saline water feed increases. In fact, at very high salt concentrations reverse osmosis should not be employed.
On the other hand, mechanical energy is applied to produce distilled water from saline water or brine in vapor compression units such as, for example, the R.C.C. brine concentrators which inherently produce distilled water of very high quality, that is, essentially salt-free water. However, some vapor compression units can process saline water of higher salt content than can be economically processed by electrodialysis or by reverse osmosis.
The practice of recovering various salts by the evaporation of sea water using solar energy and shallow ponds is well known and is centuries old. Solar stills are still used.
Efforts have been made to combine desalination systems and rely on combinations of desalination processes to make an efficient and economically acceptable system. One such system is shown in U.S. Pat. No. 3,632,505 (Nelson) issued Jan. 4, 1972. The system disclosed therein relies basically on arranging reverse osmosis and flash evaporation units essentially in parallel. High-pressure steam turbines provide the pressure for the reverse osmosis system and the high-pressure turbine discharge is used to provide the heat for the evaporation units.
Attempts have also been made to recover by-products from the desalination of brackish water. As set forth in the Fourth International Symposium on Salt -- Northern Ohio Geological Society, a process is outlined for the partial deionization of brine to produce by-products such as sodium carbonate and calcium chloride and waste such as calcium sulfate, calcium carbonate and magnesium oxide.
The combination of generation of electric power (for example, by a nuclear power plant) and desalting water has previously been proposed and studied; such plants are sometimes called "dual purpose plants." The general concept of these previously proposed dual-purpose plants is that a portion of the heat that remains in the steam after the steam drives and passes through a steam turbine (to generate electricity) is used as the heat source for a conventional saline water distillation conversion process.